Electrode system of unsymmetrical conductivity



- Nov. 22,1938. w. C(QANEEEL ET AL 2,137,428

ELECTRODE SYSTEM OF UNSYMMETRICAL CONDUCTIVITY Filed March 4, 195a VIIIIIIIIIIII/l/ll III/IIlIIIIII/III/III/IIIII/IIIII INVENTOR n 6 I444655A ETAL.

ATTORNEY Patented Nov. 22, 1938 UNITED STATES ELECTRODE SYSTEM OF'UNSYMMETRICAL CONDUCTIVITY Willem Christiaan van Geel and HendrikEmmens, Elndhoven,- Netherlands, assignors to N. V.

Philips Gloeilampenfabrieken,

Netherlands Application March 4, In Germany 7 Claims.

The invention relates to electrode systems of unsymmetrical conductivityin which one of the electrodes consists for the greater part of seleniumand is separated by a. layer of insulating material from the other, wellconducting electrode.

It is known to add metals or compounds to a selenium electrode destinedfor a dry rectifier in order to increase the rectifying effect.

The invention has for its object to increase in such an electrode systemof unsymmetrical conductivity the conductivity of the selenium and toprovide an insulating intermediate layer whose thickness may be chosenat will without said layer being acted upon by other substances.

According to the invention, the selenium electrode contains for. thispurpose conductive substances in finely divided condition so that owingto the intense intermixture they increase the conductivity of theselenium electrode itself while the insulating intermediate layer hasbeen provided as a separate layer independently of the electrodematerial.

With blocking layer photocells it has previously been suggested to addmetals or their compounds to the selenium in order to increase theconductivity. With these cells one did not obtain of course theadvantages of the additions which are afforded by the present invention,for example the exact choice of the thickness of the blocking layerbecause the provision of a particular intermediate layer is dispensedwith (for a blocking layer is automatically produced on the layer ofselenium).

Conductive materials satisfy the above mentioned requirements forfirstly, when finely divided, they highly increasethe conductivity ofthe selenium. The conductivity may be raised in this way up to ahundredfold, which affords the advantage of an appreciable reduction ofthe contact surface of the selenium electrode. Besides, the presence ofconducting substances in selenium hasno effect on the thickness of theblocking layer, even not when the upperfilm of the contact surface isremoved by vaporization from the selenium electrode for this justresults in that particles of the material added come to the surface.These particles do not increase the thickness of the blocking layer,which would be added. As far as the blocking layer is concerned, theselenium electrode has, however, a satisfactory conductivity.

Consequently, if there, are conductive sub- 56 stances the abovementioned requirement for the the case if non-conducting substances wereEindhoven,

1936, Serial No. 67,053 March 11, 1935 realization of the object of theinvention is satisfied, namely the adequate choice of the thickness ofthe blocking layer which is separately I applied.

This is of importance because thus it is possible in any particular casein which use is made of an electrode system according to the invention,to choose the minimum thickness, which materially improves the operationas the thickness of the blocking layer determines the electric fieldstrength between the electrodes. Besides, one is enabled to control thevalue of the capacity of the system by giving the blocking layer anydesired thickness.

Owing to the said favourable properties, such an electrode system isparticularly suitable for use as a detector. Owing to the satisfactoryconductivity, the area of the contact surfaces may be kept small sothat, if desired, the self-capacity of such a detector may be verysmall.

At the temperatures to which the substances to be added are heated inthe manufacture of the electrode system, they must retain theirconductivity as otherwise at the high temperature to which the selenium(in order to be melted and to be worked) and therefore also theadditions are exposed the conductive substances would changeinto-insulating compounds. Besides, the fact that the new-formedsubstances may perhaps have a less favorable influence on the in creaseof the conductivity of the semi-conductor, it is more particularly theformation of the blocking layer that plays here a part, for owing to thevaporization of the selenium the insulating substances come to thesurface as a more or less non-homogeneous layer while there still remainbehind particles of the initial conductive substance. It consequentlyremains necessary to provide a blocking layer between the contactsurfaces of the two electrodes. As in this case there are, however, twoblocking layers, namely one produced from the chemically converted addedsubstance and the one which has been applied afterwards, one obtains agreater thickness, which has a very unfavorable effect on the operationof the rectifier.

Substances suitable for being added to the selenium are, for example,the sulphides of the following metals: lead (PbS), antimony (SbzSa),copper (CuS and CuzS) the nitrides of zirconium (ZrN) and titanium (TiN)and the oxides of vanadium (V203) and cobalt (C0203).

The amount by weight of the additions ranges in general from 0.1 to 10%of the amount by weight of the selenium which is present.

In order to ensure an operation which is as advantageous as possible,the additions must have a very small size of grain, to wit a sizeranging from 1 to 10 Grains of 0.1, have in general a still morefavorable effect on the conductivity. However, the obtainment of sosmall a size of grain sometimes involves technical difllculties.

In one favorable mode of execution an insulating substance having noconstituents in common with the electrodes is utilized as theintermediate layer.

The separate application of the intermediate layer affords, as has beenmentioned before, the advantage that it is possible to controlcompletely the formation of the intermediate layer and to choose at willboth the material and the thickness and therefore the capacity. For theintermediate layer are particularly suitable, for example, collodion,shellac, paper, a $102 film or water glass. All these substances possessthe favorable property of being sufficiently resistant to the mechanicaland atmosphericinfiuences and of being sufilciently dense.

Artificial resins possess particularly favorable properties as to theiruse for the intermediate layer. These resins have so high an insulatingcapacity that a very thin film suffices, which is beneficial to thefavorable operation of the rectifier. The substances utilized for theblocking layer may be applied, for example, in a liquid or dissolvedcondition, for example, by spraying to one of the electrodes until thedesired thickness is obtained. The thickness of the blocking layerranges in general from l-to 100 1. according to the use of the electrodesystem and to the material employed.

As example of realization reference may be made to the method describedhereafter.

To molten selenium is added about 5% by weight of lead sulphide (PbS) inpulverized condit-ion (size of grain about l The mixture is spread outon an iron plate to a thickness of about 100; and then it is heated in afurnace to 200 C. for a few hours. After cooling a solution ofnitrocellulose in amylacetate is applied to the selenium surface anddried. The blocking layer thus fo med has a thickness of about 2 1;. Theother electrode is formed on said layer by applying so-called goldvarnish in which a supply conductor is taken up.

Theinvention will be explained more fully with reference to theaccompanying drawing which represents, by way of example, one embodimentthereof.

The selenium I is supported by a metallic plate 2, for example, of iron.As the blocking layer 3 is utilized collodion to which a drop of goldvarnish has been applied as the second electrode 4 of satisfactoryemitting capacity. In said drop of gold varnish is inserted a connectingwire 5. Owing to the drying of the varnish the electric supply conductorbecomes consequently secured electrically as well as mechanically to theelec-' trode of gold varnish.

We claim: 1. In an electrode system, an electrode com;-

posed of selenium mixed with an inorganic metallic compound, a secondelectrode composed of finely divided highly conductive material mixedwith a binder and a layer of insulating material intermediate the twoelectrodes.

2. In an unsymmetrical conductor device an electrodecomposed of seleniummixed with at least one of the inorganic metallic compounds in the groupincluding the sulphides of lead, copper and antimony; the nitrides ofzirconium and titanium; and, the oxides of vanadium and cobalt, toincrease the conductivity of the selenium, a second electrode composedof gold varnish adjacent the first named electrode, said two electrodesbeing separated by a layer of insulating material.

3. In an unsymmetrical conductor device, an electrode composed ofselenium intimately mixed when in a molten state with a finely dividedsubstance chosen from the group including the sulphides of lead, copperand antimony; the nitrides of zirconium and titanium; and, the oxides ofvanadium and cobalt, the proportion of the substance mixed with theselenium being from 0.1 to 10% of the amount by weight of the seleniumpresent, a second electrode composed of gold varnish adjacent the firstmentioned electrode and a thin layer of collodion separating the twoelectrodes. I v

4. The process of making an unsymmetrical conductor device whichcomprises melting selenium by the application of heat thereto, addingand intimately mixing to the molten selenium about 5% by weight of apulverized substance chosen from the group including the sulphides oflead, copper and antimony; the nitrides of zirconium and titanium; and,the oxides of vanadium and cobalt, spreading out the resulting mixtureon a plate to a thickness of about 100 and then heating the mixture toabout 200 C. for several hours, thereafter cooling the mixture, applyinga solution of nitrocellulose in amylacetate to the surface of the cooledmixture and drying the same, thereafter applying a drop of gold varnishto the coated surface and drying the varnish.

5. In an electrode system, an electrode composed of selenium mixed in aninorganic metallic compound, a second electrode composed of gold varnishadjacent to the first-named electrode and a layer of insulating materialintermediate the two electrodes.

6. A dry rectifier having two outside layers separated by a layer ofinsulating material, one of said outside layers being composed ofselenium mixed with an inorganic metallic compound, the other outsidelayer being composed of a highly conductive metallic varnish.

'7. A dry rectifier having two o'utside layers separated by a layer ofinsulating material, one of the outside layers being composed ofselenium mixed with an inorganic metallic compound in a finely dividedstate, the other outside layer being composed of gold varnish.

WILLEM CHRISTIAAN VAN GEEL. HENDRIX EMMENS.

